Creep resistant high strength zn al and zn al mg alloys

ABSTRACT

METAL FORM HAVING GOOD STRENGTH, CREEP-RESISTANCE, USEFUL DUCTILITY IS PRODUCED BY HEATING INGOT OF ZINC, 25-40 WEIGHT PERCENT ALUMINUM, AND 0-0.7 WEIGHT PERCENT MAGNESIUM AT A TEMPERATURE WITHIN THE MISCIBILITY LOOP OF THE ZINC-ALUMINUM PHASE DIAGRAM; QUENCHING; REHEATING TO 225*-325*C. AND FORMING AT THIS TEMPERATURE. FOR ADDITIONAL IMPROVEMENT, THE FORM IS SOLUTION-TREATED, QUENCHED AND HEAT STABILIZED.

United States Patent ()flice US. Cl. 148-12.7 9 Claims ABSTRACT OF THE DISCLOSURE Metal form having good strength, creep-resistance, useful ductility is produced by heating ingot of zinc, 2540 Weight percent aluminum, and O-0.7 weight percent magnesium at a temperature within the miscibility loop of the zinc-aluminum phase diagram; quenching; reheating to '225325 C. and forming at this temperature. For

additional improvement, the form is solution-treated, quenched and heat stabilized.

This invention relates to preparing and fabricating zincbase alloys.

Zinc-aluminum (Zn-Al) alloys of near eutectoid composition have been the subject of substantial interest. Much of this interest stems from the ability of these alloys to exhibit superplastic behavior (exaggerated ductility) when quenched from above the Zn-Al eutectoid temperature of 275 C, and then deformed not far below 275 C. However, poor tensile and creep strengths have been serious detriments to wider use of wrought zinc alloys.

We have now discovered that the combination of certain heating and forming temperatures for treatment of an alloy consisting essentially of zinc, 25-40 Weight percent aluminum and to 7 weight percent magnesium results in the fabrication of zinc-base alloys which exhibit useful ductility or excellent creep resistance or high strength or a combination of these properties. Broadly, the process comprises heat-treating (conditioning) the alloys at a temperature within the miscibility loop above the eutectoid temperature (275 C.) of the Zn-Al phase diagram which diagram is shown in Constitution of Binary Alloys, Hansen and Anderko, page 149, McGraw-Hill 1958. Such conditioning is carried out for a time suflicient to essentially homogenize the phase structure throughout the piece. Thereafter the alloy is quenched, reheated to about 225-325 C., and formed at this temperature. The form is allowed to slowly cool (air-cool) As used throughout the specification and claims, the term forming or formed includes working.

To further enhance its properties, the form is reheated (solution-treated) at a temperature within the above described miscibility loop for a time suflicient to essentially homogenize the phase structure throughout the piece. Thereafter the piece is quenched and again reheated for the purposes of stabilizing or aging. These post-forming treatment steps are most suitable for magnesium-containing alloys.

In the case of magnesium-free alloys, solution treatment is carried out at a temperature of about 276375 C., followed by quenching to 225--274 C., and heating at this latter temperature for a period to essentially transform the 3,764,402 Patented Oct. 9, 1973 high temperature phase structure which existed above 275 C.

It is therefore an object of the present invention to produce zinc-based alloys possessing such properties as useful ductility, high tensile strength or excellent creep resistance.

Another object is to produce such alloys having high tensile strength coinciding with excellent creep resistance and useful ductility.

A further object is to improve creep resistance and/or tensile strength of zinc-based forms.

A still further object is to reduce required forming pressures.

Other objects and advantages will be obvious from the following more detailed description of the invention taken in conjupnction with the drawing in which:

FIG. I1 is a portion of the Zn-Al phase diagram; and

FIG. 2 is a graph illustrating one of the properties of the product of the present invention.

In the practice of the present invention, the alloy of the desired composition is obtained by preparing a melt of the requisite ingredients, and casting ingots therefrom. The alloy is essentially composed of (a) zinc, (b) 25-40 weight percent aluminum, and (c) 0-0.7 weight percent magnesium, preferably 0.05-0.3 weight percent magnesium. If desired the ingots are machine cut or trimmed to a convenient handling size. Furthermore, although not essential, the ingots may be given a heat treatment for several hours above 275 C. for the purpose of chemical homogenization.

After final preparation of the ingot, which may include machining, chemical homogenizing, etc., the alloy is now ready for the special heat treatments and forming temperature steps of the process of the present invention. In the first of these steps the alloy is condition heat treated within the miscibility loop of the Zn-Al phase diagram. -In other words, the heating temperature falls within the miscibility loop with regard to the Zn-Al composition of the piece being treated. FIG. 1 illustrates the miscibiilty loop (shaded area) of the Zn-Al phase diagram. As can be seen from the FIG. 1, the lower limit of the heat conditioning range is just above 275 C., while the upper limit is dependent upon the ratio of zinc to aluminum in the piece being heated; and the upper limit ranges from about 300 C. (25% aluminum) to about 350 C. (40% aluminum).

Such heat conditioning is carried on until the phase structure is essentially homogenized throughout the piece which usually requires 4 hour or more. After the heating period, the alloy is rapidly quenched to about room temperature (e.g., 20-30 C.) with, for example, water.

Such preliminary treatment develops fine graded transformed structures in the alloy which favorably influence both subsequent forming and mechanical properties of the final product.

Next, the alloy is heated to a forming temperature of about 225 -325 C., preferably 250-300 C., thereafter formed and subsequently allowed to slowly cool (air-cool) to room temperature. At a temperature within the range of 250-275 C., the energy required for forming is considerably less than the energy required at forming temperatures either above 275 C. or below 250 C. This is creep resistance than those extruded below. Further, alloys with magnesium display much better creep resistance than magnesium-free alloys. For a relatively high stress of 12,000 p.s.i., the creep resistance of these Zn-Al-Mg alloys is very good.

Whether formed above or below the eutectoid temperature the form exhibits high tensile strength with useful ductility. Exemplary test results in this regard are shown in the following Table 2. Ductility is represented in this table by the ability of the test sample to be elongated or reduced in cross-sectional area.

TABLE 2 Tensile properties of Znpercent Al and 82013 3?) p ercent Al-Mg alloys as extruded at 250 or Extrusion Tensile Yield Elonga- Reduction temperastrength, strength, tion, in area, Alloy ture, C. p.s.l. I p.s.i. 5 percent percent Zia-30 percent Al 250 23, 700 21, 000 92 95 D 500 30,600 27, 700 32 53 Zn-30 p Mg-.. 250 60, 800 56, 500 22 74 Do 300 59, 400 56, 000 9 16 211-30 percent Al0.3 percent Mg 250 60, 000 55, 500 22 74 Do 300 62, 400 57, 700 11 58 Zn-30 percent Al-OJ percent Mg 250 61, 400 56, 700 12 34 Do 300 63, 800 58, 600 11 31 movement) 1 At maximum load. 3 0.2 percent ofiset method.

(e) Billets were conditioned at 300 C. for 2 hours, and water quenched to about room temperature;

(f) Conditioned billets were extruded to inch diameter at an extrusion speed (rod exit speed) of 2-3 feet/ minute.

It can be seen from the figure that, whatever pre-forming heating procedure is employed on the alloys of the present invention, forming pressures markedly increase at above the eutectoid (275 C.) and below 250 C. With adjusted heating time before forming, magnesium-containing alloys can be as readily extruded as magnesium-free alloys.

Although forming temperatures above 275 C. require higher forming pressures in comparison to forming temperatures of 250275 C., magnesium-containing alloys formed at above 275 C. exhibit excellent creep resistance in the as-formed condition. Exemplary comparative test results are shown in Table 1. These and the following examples are for zinc-30 percent aluminum alloys containing 0-0.7 percent magnesium. In the tables, the lower the creep rate, the higher the creep resistance.

"The inch diameter rods were extruded as in FIG 2; for creep specimens, the rods were machined to flat 7-inch long slats, .08 inch in thickness, inch in width at the shoulders of each slat and havingawidth of 0.20 inch at a reduced two-inch gage section; the steady-state creep rates (percent elongation per day) were measured in the reduced section at 90+3 F. under a stress of 12,000 p.s.i.

It can be seen from Table 1 that those alloys extruded above the eutectoid (275 C.) possess markedly better In addition to showing the desirable properties of magnesium-containing alloys, Table 2 illustrates the low tensile strength and high ductility of magnesium-free alloys in the as-extruded condition of the process of the present invention.

Special post-forming heat treatments further improve the properties of the alloys of the present invention, particularly those formed below the eutectoid temperature. These procedures includes (a) heating (solution-treating) the form at a temperature within the miscibility loop for a period suflicient to essentially homogenize the phase structure throughout the piece, usually l4 hour or more; (b) rapidly quenching (e.g., water-quenching) to about room temperature; (c) reheating (aging or stabilizing) to about 75 to 175 C., preferably about to C., for a time sufiicient to essentially homogenize the phase structure throughout the piece, usually about 4 to 2 hours; and (d) thereafter cooling (such as air-cooling) to room temperature. Such treatment significantly improves the creep resistance of magnesium-containing alloys which have been formed either above or below the eutectoid, and most particularly those formed below. In addition, such treatment significantly improves the tensile strength of magnesium-containing alloys which have been formed above the eutectoid temperature, and to a lesser extent those formed below. Ductility in both cases is somewhat decreased by the post-forming treatments. These results are exemplified in the following tables:

TABLE 3 Creep rate, percent/day Extrusion temperature, G 250 300 Alloy:

Zn-30 percent A1 37 21. 9 Zn-30 percent Al-(Ll percent M 0. 0743 .0024]. Zn-30 percent A10.3 percent Mg 0657 00239 Zn-30 percent Al-OJ percent Mg 0674 1 See footnote at and 0t able 1,

' Creep g TABLE 4 ila tesgi Zn30-perce'nt Aland Zn'30 percent Al-Mg alloys extruded at 25 or C., then solution-treated at 350 C. for 2 hrs, quenched and aged at 100 O. my, hr.

Creep rate, percent/day 1 see footnote at end of Table 1.

l 6 sium-containing alloys. In this embodiment the form is heated (solution-treated) at about 276 to 375 C., preferably 325 to 350 C., for about hour or more, or long enough to essentially homogenize the elevated temperature structure throughout the piece, is cooled directly (without cooling below and reheating) to about 225 to 274 C., preferably about 250 C., and thereafter held at this latter temperature for about A to 4 hours, or for a period needed to transform the high temperature structure that existed above 275 C.

Finally, it is cooled to room temperature. It should TABLE 6 Tensile properties 1 of Znpercent Al and Zn30 percent .Al-Mg alloys extruded at 275 C., then solution-treated at 350 C. for 2 hr., quenched, and aged at 150 C. for 54 hr.

1 See footnotes at end 0! Table 2.

Referring first to Tables 3 and 4, it can be seen, in comparison to Table 1, that the post-forming heating procedure vastly reduces creep in magnesium-containing alloys extruded below the eutectoid (275 C.). With further regard to magnesium-containing alloys extruded below the eutectoid, a higher solution-treating temperature (350 C.) obtains the best results with regard to creep resistance, but as discussed later, not with regard to accompanying ductility.

Referring now to Tables 5 and 6, it can be seen, in comparison to Table 2, that for magnesium-containing alloys extruded at or above the eutectoid (275 C.), there is a substantial increase in tensile strength by post-forrning treatment. However, as shown in Table 6, at a solution-treatment temperature of about 350C., the increase in tensile strength is not accompanied with useful ductility. It can also be seen from these tables that after postforming treatment, magnesium-containing alloys extruded below'the eutectoid have considerably lower strength and higher ductility than those extruded above the eutectoid. Ductility is lowest in the alloy containing 0.7 percent magnesium. Magnesium-free alloys display high ductility and low strength before and after post-extrusion heat treatments.

For magnesium-free alloys formed above the eutectoid, there is an alternative post-forming heating procedure which increases tensile strength and vastly improves creep resistance although magnesium-free alloys so treated still do not possess the degree of creep resistance of magnebe stressed that this treatment is superior for magnesium- 7 TABLE 5 Tensile properties 1 oi-Zn30 percent Al and Zn-30 percent Al-Mg alloys extruded at 250 W or'285". C., then solution-treated at 300 C., quenched, and aged at 100 C.

v I Extrusion Tensile Yield Elon- Reduction I temperastrength, strength, gation, in area, :Alloy ture, C. p.s.i. p.s.i. percent percent zn-ao eic ritjm 25o 26,500 22, 700 72 5 Do 1 285 24, 700 21,000 60 99 Zn-30 percent Al-O. p

Mg 250 67, 600 62, 100 11 45 Do v 285 81,100 78, 500 8 18 Zn-30 percent .Al-OB percen Mg 250 68,000 61,900 16 43 285 83, 200 79,900 8 20 1 See footnotes 1, 2, and 3 at end of Table 2. ll Alloys extruded at 250 C. were solution-treated for 3 hours and aged for hour. 1 Alloys extruded at 285 C. were solution-treated for 2 hours and aged for 1 hour.

free alloys but inferior to previous post-forming treatments for the magnesium-containing alloys. Test results exemplifying this procedure are shown in the following tables:

TABLE 7 Creep rates of Zn-30 percent Al and Zn-30 percent Al-Mg alloys extruded at 300 C., then solution-treated at 350 C for hr. and cooled directly to 50 C. and held 3 hr. for isothermal transformation Creep rate, Alloy percent/day Zn-30 percent Al 0. 358 Zn30 percent Al0.1 percent Mg 0176 Zn-30 percent Al0.3 percent Mg 0106 Zn-30 percent Al0.7 percent Mg 0255 1 See footnote at end of Table 1.

TABLE 8 Tensile properties of Zn-30 percent Al and Zn-30 percent Al-Mg alloys extruded at 300 C., then solution-treated at 350 C. for 2 hr. and cooled 1 See footnotes 1, 2, and 3 at end of Table 2.

It can be seen from Tables 7 and 8 that not only are the strength and creep resistance of magnesium-free alloys improved by such post-extrusion treatment, but the magnesium-free alloys still exhibit substantially high ductility.

In the practice of the present invention in addition to extrusion, forming can be accomplished by rolling, forging etc. Generally, due to their properties, the magnesiumcontaining formed products of the present invention are not suitable for further forming steps at near room temperature.

All the heating steps of the invention can be carried out while the alloy is in contact with air, although other methods such as oil baths etc., could be employed.

In the practice of the present invention, to obtain useful ductility simultaneously with high strength and creep resistance, it is preferred to employ 0.05 to 0.3 percent magnesium in the alloy. Higher magnesium additions result in generally lower ductility although strength remains high. The presence of lower amounts of magnesium adjusts properties to higher ductilities at the expense of strength, which in some instances may be desired.

We claim:

1. A process for producing metal forms comprising:

(a) preparing an alloy consisting essentially of zinc, 25- 40 weight percent aluminum and 0."7 weight percent magnesium;

(1:) heat conditioning said alloy at a temperature within the miscibility loop of the zinc-aluminum phase diagram;

(0) rapidly quenching said alloy to about room temperature;

(d) reheating said quenched alloy to a temperature of about 225 -325 C.;

(e) forming said reheated alloy; and

(f) slowly cooling said form to about room temperature.

2. The process of claim 1 further including solution treating said form at a temperature within said miscibility loop to essentially homogenize phase structure; quenching; and aging at about 75 to 175 C. to homogenize and stabilize said phase structure throughout said form.

3. The process of claim 1 wherein said forming temperature is about 250-300 C.

4. The process of claim 1 wherein said alloy contains 8 magnesium, and said forming temperature is above 275 C. a

5. The process of claim 1 wherein said alloy contains ODS-0.3 weight percent magnesium.

6. The process of claim 2 wherein said forming temperature is about 250-300 C.

7. The process of claim 2 wherein said forming temperature is above 275 C., and said alloy contains magnesium.

8. The process of claim 2 wherein said alloy contains ODS-0.3 weight percent magnesium.

9. The process of claim 2 wherein said aging temperature is about to C.

References Cited UNITED STATES PATENTS 3,676,115 7/ 1972 Hare et al 75178 AM 1,945,288 1/ 1934 Morell 75178 AM 2,008,529 7/ 1935 Werley 75178 AM 3,420,717 H1969 Fields et a1. 148-115 R 3,632,454 3/1970 Marshall 148-115 R FOREIGN PATENTS 4,822 12/1953 Germany 75178 5,659 2/1954 Germany 75-178 CHARLES N. LOVELL, Primary Examiner US. Cl. X.R. 148-2, 11.5 R 

